1. Field of the Invention
The present invention relates to a matrix substrate, a liquid crystal device using the substrate, and a method for producing the matrix substrate.
2. Related Background Art
The world of today is a multimedia world, and equipment for communication by image information is becoming more and more important. Among others, the liquid crystal display devices are drawing attention because of their slimness and low power consumption. The liquid crystal display industry has grown to be a basic industry comparable to the semiconductor industry. Liquid crystal display devices are mainly used for 10-inch notebook-size personal computers at present. It is expected that liquid crystal display devices of larger screen sizes will be used not only for personal computers, but also for workstations and televisions for home use in the future. With an increase in screen size, however, manufacturing equipment becomes expensive, and, in addition, electrically exacting characteristics are demanded for driving of such large screens. The manufacturing cost will thus increase abruptly in proportion to the square to cube of the size with increasing screen size.
Recently, attention has been drawn to a projection method for preparing a compact liquid crystal display panel and optically enlarging a liquid crystal image to display an enlarged image. This is because the microstructure tendency of semiconductors permits decrease in size, improvement in the characteristics, and decrease in the cost, similar to the scaling rule to improve performance and cost. From these aspects, in the case of the liquid crystal display panel of the TFT type, TFTs have to be compact and have sufficient driving force, and transition is now occurring from TFTs using amorphous Si to those using polycrystal Si. Video signals of the resolution level conforming to the NTSC system, etc. used in ordinary televisions do not require so quick processing.
This allows not only the TFTs but also peripheral driving circuits such as shift registers or decoders to be made of polycrystal Si, whereby liquid crystal display devices can be constructed in a monolithic structure of a display region and a peripheral driving circuit region. Polycrystal Si is inferior to single crystal Si, however. For realizing high definition televisions having a higher resolution level than the NTSC system or display of the XGA (eXtended Graphics Array) or SXGA (Super eXtended Graphics Array) class in resolution standards for computers by polycrystal Si, a shift register needs to be composed of a plurality of segments. In this case, noise, called ghost, appears in the display region at portions corresponding to borders between the segments. A solution to this problem is desired in this field.
On the other hand, focus is also drawn to display devices using a single crystal Si substrate, which can realize extremely high driving force as compared to display devices of the monolithic structure of polycrystal Si. In this case, the transistors of the peripheral driving circuitry have sufficient driving force, and thus the divisional driving described above is not necessary. This solves the problem of the noise and the like.
Even with either of these polycrystal Si and single crystal Si, a reflection-type liquid crystal device can be provided in such a structure that a reflection-type liquid crystal element is formed by connecting the drain of each TFT to a reflective electrode and interposing the liquid crystal between the reflective electrodes and a transparent common electrode and that horizontal and vertical shift registers for scanning of the liquid crystal element are formed on the same semiconductor substrate. The applicant of the present application filed Japanese Laid-Open Patent Application No. 9-73103 to disclose the reflection-type liquid crystal device using a substrate of polycrystal Si or single crystal Si. The invention disclosed in the application solves the following problems: when light is incident to a pixel electrode, the incident light is scattered in all directions by unevenness of the surface, and reflection efficiency of light thus becomes very small; and this unevenness of surface becomes the cause of alignment failure in a rubbing step of the alignment layer in a liquid crystal packaging process, and this results in causing alignment failure of the liquid crystal, so as to degrade the display image due to lowering of contrast.
In the Japanese Laid-Open Patent Application No. 9-73103, the pixel electrode surface is polished by chemical mechanical polishing (hereinafter referred to as “CMP”). This smooths the pixel electrode surface like a mirror-finished surface and makes the whole pixel electrode surface be in a common plane. This prevents the irregular reflection and alignment failure caused by the unevenness and thus permits display of an image with high quality.
A method for producing an active matrix substrate, disclosed in the Japanese Laid-Open Patent Application No. 9-73103, will be described referring to FIGS. 39A to 39E and FIGS. 40F to 40H. FIGS. 39A to 39E and FIGS. 40F to 40H show a pixel section and, at the same time as a step of forming the pixel section, the peripheral driving circuits such as the shift registers for driving the switching transistors in the pixel section can also be made on the same substrate.
An n-type silicon semiconductor substrate 201 with an impurity concentration of not more than 1015 cm−3 is locally thermally oxidized to form LOCOS 202, and, with the LOCOS 202 as a mask, ions of boron are implanted in a dose of about 1012 cm−2 to form PWL 203, which represents p-type impurity regions with an impurity concentration of about 1016 cm31 3 . This substrate 201 is again thermally oxidized to form gate oxide film 204 having an oxide film thickness of not more than 1000 Å (FIG. 39A).
Gate electrodes 205 made of n-type polysilicon doped with phosphorus of about 1020 cm−3 are formed, and thereafter ions of phosphorus are implanted in a dose of about 1012 cm−2 over the entire surface of substrate 201 to form NLD 206, which represents n-type impurity regions having an impurity concentration of about 1016 cm−3. Subsequently, using a patterned photoresist as a mask, ions of phosphorus are implanted in a dose of about 1015 cm−2 to form source and drain regions 207, 207′ having an impurity concentration of about 1019 cm−3 (FIG. 39B).
PSG 208, which is an interlayer film, is formed over the entire surface of substrate 201. This PSG 208 can be replaced by NSG (Nondoped Silicate Glass)/BPSG (Boro-Phospho-Silicate Glass) or TEOS (Tetraethoxy-Silane). The PSG 208 is patterned to form contact holes immediately above the source and drain regions 207, 207′, Al is evaporated by sputtering, and thereafter the Al layer is patterned to form Al electrodes 209 (FIG. 39C). In order to improve ohmic contact characteristics of the Al electrodes 209 with the source and drain regions 207, 207′, a barrier metal such as Ti/TiN is desirably placed between the Al electrodes 209 and the source/drain regions 207, 207′.
Plasma SiN 210 is deposited in a thickness of about 3000 Å over the entire surface of substrate 201, and then PSG 211 is deposited in a thickness of about 10000 Å thereon (FIG. 39D).
Using the plasma SiN 210 as a dry etching stopper layer, the PSG 211 is patterned so as to leave only separating regions between pixels, and thereafter the plasma SiN 210 is patterned by dry etching to form through holes 212 immediately above the Al electrodes 209 in contact with the drain regions 207′ (FIG. 39E).
Then a pixel electrode layer 213 is deposited in a thickness of not less than 10000 Å on the substrate 201 by sputtering or EB (Electron Beam) evaporation (FIG. 40F). This pixel electrode layer 213 is a metal film of Al, Ti, Ta, W, or the like, or a compound film of these metals.
The surface of the pixel electrode layer 213 is then polished by CMP (FIG. 40G).
An alignment layer 215 is further formed on the surface of the active matrix substrate formed by the above steps, and the surface thereof is subjected to an alignment process such as a rubbing process. The substrate is bonded through a spacer (not illustrated) to an opposite substrate, and then liquid crystal 214 is injected into the gap between them to complete a liquid crystal element (FIG. 40H). In this case, the opposite substrate is composed of a color filter 221, a black matrix 222, a common electrode 223 of ITO or the like, and an alignment layer 215′ on a transparent substrate 220.
A driving method of this reflection-type liquid crystal element is as follows. By the peripheral circuits such as the shift registers formed in an on-chip fashion on the substrate 201, a signal potential is applied to a source region 207 and, at the same time, a gate potential is applied to the associated gate electrode 205 to switch the switching transistor of that pixel on, thereby supplying signal charge to the drain electrode 207′. The signal charge is accumulated in a capacitor of the depletion layer of the pn junction created between the drain region 207′ and the PWL 203 to give a potential through the Al electrode 209 to the pixel electrode 213. When the potential of the pixel electrode 213 reaches a desired value, the potential applied to the gate electrode 205 is switched off to turn the pixel switching transistor off. Since the signal charge is accumulated in the pn junction capacitor part described above, the potential of the pixel electrode 213 is fixed before the pixel switching transistor is next driven. This fixed potential of the pixel electrode 213 drives the liquid crystal 214 encapsulated between the substrate 201 and the opposite substrate 220 illustrated in FIG. 40H.
In the reflection-type liquid crystal display device described above, the surface of the pixel electrode 213 is smooth, and the insulating layer is filled in the gap between adjacent pixel electrodes; therefore, the surface of the alignment layer 215 formed thereon is also smooth without unevenness. This prevents the decrease of light utilization efficiency due to scattering of incident light, the decrease of contrast due to inappropriate rubbing, and occurrence of bright lines due to a horizontal electric field raised by a level difference between the pixel electrodes. Therefore, the quality of a display image is improved.
The liquid crystal display device has excellent characteristics as described above, but studies by the present inventors have shown that the device is still susceptible to further improvement.
Specifically, the formation of the pixel electrode layer 213 is carried out by deposition of the electrode material at a high temperature and further thermal treatment so as to sufficiently densely pack the electrode material in recessed portions, which raises a possibility that residue or gas or the like will evolve and heat brings about reaction thereof.
The heat could bring about reaction between the pixel electrodes and the base layer thereof at the corners of the bottom of the pixel electrodes (or at the base of the insulating separation regions between the pixel electrodes) in some cases. Particularly, in the case where there are the pixel electrodes and a shielding layer or an electroconductive layer for formation of a capacitor placed through an insulating layer below the pixel electrodes, there is a possibility that the state of the insulating layer becomes imperfect at the base of the separation regions between the pixel electrodes, whereby the pixel electrodes react with the electroconductive layer or whereby leak current flows between them.
This will be described in more detail referring to FIG. 41 and FIG. 42.
FIG. 41 is a sectional view of a reflection-type liquid crystal display device, which is similar to that illustrated in FIG. 40H, but which clearly shows the shielding layer 7 provided in the matrix substrate 410. In FIG. 41, the liquid crystal display device is constructed in a structure in which the liquid crystal layer 14 is interposed between the matrix substrate 410 and the opposite substrate 420. The opposite substrate 420 is constructed of a common electrode 15 and an anti-reflection film 20 provided on a transparent substrate 16.
In the matrix substrate 410, numeral 10 represents the source electrode of each transistor being a switching device of pixel electrode 12, and numeral 11 represents the drain electrode connected to the pixel electrode 12. Numeral 7 indicates the shielding film made of an electroconductive metal material. The shielding film 7 is separated from the pixel electrodes 12 by insulating film 21. Numeral 9 designates an electrically insulating member for separating the pixel electrodes from each other.
In the liquid crystal panel illustrated in FIG. 41, the pixel electrode 12 and shielding film 7 compose a capacitor through the insulating film 21, which works as a storage capacitor during application of an electric field to the liquid crystal.
FIG. 42 is an enlarged view of the part near the insulating member 9 for separation of the pixel electrodes in FIG. 41.
As illustrated in FIG. 42, a discontinuity surface is likely to be formed at the root part of the insulating member 9 during deposition of the insulating film 21 because of the nearly vertical configuration of the insulating member 9. This increases the possibility that dielectric breakdown may occur with this discontinuity surface as a leak path at a lower voltage than the withstand voltage against dielectric breakdown of the insulating film 21.
Since the separation region part 9 between the pixel electrodes 12 also serves as a stopper on the occasion of CMP or the like, it needs to have some mechanical strength, and there are some cases where further increase of the mechanical strength is demanded of the separation region part between the pixel electrodes in view of the tendency toward higher density and the like.